While the US has imported crude oil as energy carrier for several decades, natural gas demand has mostly been met from domestic supplies. However, domestic supplies of natural gas are beginning to diminish due to increasing demand from industrial, residential, and/or electric utilities consumers. This situation is further compounded by the replacement of older power plants with new “clean-burning” natural gas power plants. Consequently, LNG import has become economically increasingly attractive, and existing LNG regasification facilities are currently being expanded as well as new regasification facilities are being added. Conventional LNG regasification facilities typically require an external heat source such as an open rack seawater vaporizer, a submerged combustion vaporizer, an intermediate fluid vaporizer (e.g., using a glycol-water mixture), or ambient air vaporizers. LNG vaporization is a relatively energy intensive process and typically requires heating energy of about 3% of the energy content in LNG.
Combined cycle power plants use both steam and gas turbines to generate power, and generally achieve higher energy conversion efficiency than gas or steam only plants. Power plants may be coupled with LNG regasification, as described in U.S. Pat. Nos. 4,036,028 and 4,231,226 to Mandrin and Griepentrog, respectively. Similar configurations are reported in published U.S. Pat. App. No. 2003/0005698 to Keller, EP 0 683 847 and EP 0 828 925 to Johnson et al., WO 02/097252 to Keller, and WO 95/16105 and WO 96/38656 to Johnson. In such known configurations, heat for regasification of LNG is provided by a heat exchange fluid, which is in thermal exchange with the turbine exhaust or a combined cycle power plant.
While some of the above configurations provide reduction in the energy consumption for LNG regasification, gains in power generation efficiencies are often not significant. Still further, and among yet other difficulties, the heat transfer in some of these configurations is limited by the freezing point of the heat transfer medium. Moreover, while the refrigeration content of the LNG is utilized to at least some degree, electric or other power is not extracted from such configurations.
In further known configurations, as described in EP 0 496 283, power is generated by a steam expansion turbine that is driven by a working fluid (here: water) that is heated by a gas turbine exhaust and cooled by a LNG regasification circuit. While such a configuration increases efficiency of a plant to at least some degree, several problems remain. For example, cryogenic refrigeration content of the LNG is typically unused as the freezing point of water or a water glycol mixture is relatively high. To overcome difficulties associated with high freezing temperature, non-aqueous fluids may be employed as a working fluid in the Rankine cycle power generation. Such a configuration is exemplified in U.S. Pat. No. 4,388,092 to Matsumoto and Aoki, in which the fluid is provided by a distillation column operating in a batch distillation cycle. However, the operation of such batch system is difficult and complex. Moreover, most of such Rankine cycle processes fail to utilize the full temperature range in LNG regasification. In still further closed cycle operations as described in EP 0 009 387 to Mak, WO 99/50536 to Minta, or WO 99/50539 to Bowen, closed cycle processes utilize the cold content in LNG or PLNG to produce power. While such conceptually relatively simple processes provide at least some energy from the LNG cold, various disadvantages similar to those provided above remain.
Where LNG is processed to a typically lean LNG with lower heating value, the LNG may be employed as a working fluid in an open cycle within the fractionation processes as described by J. Mak in WO 2004/109180 and WO 2004/109206. In such configurations a portion of the flashed LNG is pumped to pressure and then expanded after a significant portion of the cold has been extracted. The so expanded LNG is then fed to a demethanizer for processing. Such processes typically provide significant energy savings in the production of lean LNG with power co-production. Moreover, these processes also allow production of relatively pure ethane and heavier hydrocarbon components from the rich LNG. However, such configurations are typically limited to LNG processing with nominal power generation requirement and fail to lend themselves to full utilization of the LNG cold in power generation in LNG regasification facilities.
Therefore, while numerous processes and configurations for LNG utilization and regasification are known in the art, all of almost all of them suffer from one or more disadvantages. Thus, there is still a need to provide improved configurations and methods for LNG utilization and regasification.